1. Field of the Invention
The present invention relates to the art of a control system for a hybrid drive unit in which a rotational speed or a power generation amount of a motor-generator serving as a prime mover is changed in accordance with a speed ratio of a transmission connected to an output side of an engine.
2. Discussion of the Related Art
JP-A-2011-255889 describes a hybrid drive system having a power distribution device comprised of a first rotary element to which an engine power is applied, a second rotary element connected with a first motor-generator, and a third rotary element connected with an output member connected with a second motor generator. In the hybrid drive system taught by JP-A-2011-255889, an engine speed is changed by controlling a speed of the first motor-generator. In the hybrid drive system of this kind, an engine operating point can be controlled in a fuel efficient manner by controlling a speed of the first motor generator. To this end, the first motor-generator is operated as a motor, and a second motor-generator is operated as a generator at high speed range, but a power circulation may be caused. According to the teachings of JP-A-2011-255889, therefore, a transmission is interposed between the engine and the power distribution device, and a speed ratio of the transmission is changed according to a vehicle speed, so as to prevent the first motor-generator to be operated as a motor at high vehicle speed.
In the hybrid drive system taught by JP-A-2011-255889, the engine speed is changed responsive to a change in the speed of the first motor-generator. That is, the engine can be cranked and stopped by the first motor-generator. When starting or stopping the engine, rotational speeds of rotary members including the first motor-generator are changed. In this situation, both motor-generators generate powers, or one of the motor-generators regenerates a power in an amount of change in an inertial energy changed in accordance with the change in the rotary members. Therefore, if a large amount of electric power is generated under a condition that a battery is charged sufficiently, the battery may be overcharged and damaged. In contrast, if the electric power is discharged overly from the battery to start or stop the engine under the condition that a state of charge of the battery is insufficient, the battery may also be damaged by such an over discharging.
The present invention has been conceived noting the foregoing technical problems, and it is therefore an object of the present invention is to provide a control system for a hybrid drive unit to prevent an overcharging and an over discharging of an electric storage device when starting and stopping an engine.